The Leinster three-quarter suffered his injury blow during Ireland's open training session at the Aviva Stadium on Friday, and could be out of action for two months.

The 28-year-old impressed off the bench in Ireland's 43-20 World Cup quarter-final defeat to Argentina, and would have now been hoping to force his way into boss Joe Schmidt's starting line-up.

Now though the 34-cap flyer will be unable to help Ireland in their quest to retain the Six Nations trophy for an unprecedented third year in succession.

"Luke Fitzgerald has been ruled out of the Six Nations with a medial ligament injury to his knee," said Ireland's team manager Mick Kearney.

"He is expected to be out of action for six to eight weeks and at this stage no replacement has been called in.

"He was just doing a tackling drill at the Aviva on Friday, his foot got caught in the grass.

"It was fairly innocuous but very unfortunate and he's very unlucky to be out of the Six Nations."

Ireland are already without Ulster wing Tommy Bowe, still a long-term casualty with knee trouble.

Head coach Schmidt can still call on Andrew Trimble, Dave Kearney and Keith Earls for the wing, with Simon Zebo able to feature out wide or at full-back.

"He was very upset and probably feared the worst," said Mick Kearney.

"He'd had a very good injury run recently, had a great World Cup and was flying for Leinster."

Flanker Sean O'Brien admitted one of Ireland's toughest Six Nations challenges will be to "fill the void" left by talismanic captain Paul O'Connell's Test retirement.

Rory Best has taken Ireland's reins, with the likes of Leinster back-rower O'Brien likely to hold strong influence in the dressing room.

Ireland open their Six Nations defence against Wales at the Aviva Stadium on Sunday, with O'Brien accepting the entire squad must adapt in lock O'Connell's absence.

"It's kind of good - he doesn't stare at us and intimidate us any more!" joked O'Brien, when asked about life without O'Connell.

"But it's obviously different not having Paulie around with the type of player he is - and it's up to other lads to fill the void, stand up and be counted. Everyone's stepping up, there's been a good mix.

"Rory has passed on a lot of good messages over the last few weeks.

"The leadership group have to move on very quickly and drive the group, and I think that's happening in the last two weeks."

South Africa-born CJ Stander is primed to make his Ireland debut across the Six Nations after completing his three-year residency qualification.